1. Field of the Invention
The present invention generally relates to an optical amplifier with spectral gain monitoring functions and in particular to a compact erbium-doped fiber amplifier (EDFA) with gain spectrum and optical performance dynamically controlled.
2. Status of the Prior Art
The past decade has witnessed a rapid growth in the volume of high-speed data traffic carried over national and international communication networks. This growth has been driven principally by the dramatic increase in the wide use of the Internet and commercial data networks. This tremendous amount of worldwide data traffic volume requires fiber-optic communications networks having multi-gigabit transmission capacities with highly efficient cross-connect links. To this end, in the field of fiber-optic technology, products have been developed for multi-carrier transmission over a single fiber thereby multiplying the amount of information capacity transmitted over a single carrier system. By assembling several individual data signals of different wavelengths into a composite multi-channel signal transmitted on a single fiber(i.e., wavelength division multiplexing (WDM)), it is possible for multiple users to share a common fiber-optic link and thereby realize high throughput.
To assemble the multi-channel signals, a multiplexing device (MUX) is employed at the transmitting end that combines the multiple light-wave signals from several sources or channels of different wavelengths into a single composite signal. The center wavelengths of the signals must be properly spaced and have pass bands well defined in order to avoid cross-talk between channels. For example, the well-accepted industrial standard is a channel spacing of 100 GHz (0.8 nm in 1.55 xcexcm window) centered at the ITU grid wherein each signal channel has a pass bandwidth of 0.3 nm at 0.5 dB down power level. The multiplexed signal is then transmitted on a single fiber-optic communications link. At the receiving end, a demultiplexing device (DEMUX) separates the composite signal received from the fiber link into the original channel signals, each of which is a single signal channel centered at the ITU grid. Such dense wavelength division multiplexing (DWDM) technology dramatically increases the information-carrying capacity that is transmitted on a single carrier fiber. For example, a 40-channel 100 GHz DWDM system with a 10 Gb/s transmission rate can transmit 400 Gb/s data in the C-band (1528-1563 nm). The number of channels deployed in long-haul DWDM systems is rapidly increasing to beyond 100 channels over the C-band and L-band (1575-1610 nm).
In optical networks having a large number of channels, the stability of the channels (both in terms of the amplitude and wavelength) is critical. The stability of channels in optical networks is largely dependent on the operational characteristics of the optical amplifiers, optical transmitters, and network architecture.
As the multi-wavelength signals propagate along the optical fibers, the powers of the signals are gradually decayed due to the presence of insertion, distribution, and transmission losses. To boost the signal powers, optical amplifiers are employed periodically to compensate for the power loss. Optical amplifiers receive one or more optical signals and simultaneously amplify all wavelengths. This is a significant advantage of multi-wavelength fiber systems over regenerators. However, not all channels are amplified by the same factor because the gain spectrum of the optical amplifier is not uniform. For example, the gain spectrum of an EDFA has well-known asymmetrical twin peaks due to a luminescent spectrum caused by the fine structure of the energy levels. Because the gain spectrum is not flat, a power deviation exists between the amplified signals that corresponds to the different wavelengths. Though a gain flattening technique can resolve this, it is important to monitor power fluctuations of individual channels, rather than aggregate power.
It is also well known that the wavelength and amplitude of the light emitted by the lasers tends to vary as the lasers age and as the operational temperature of the lasers changes. As the number of channels deployed in a WDM optical network increases, wavelength drifts are more likely to result in interference between channels because the channel spacing is narrower. As a result, wavelength drifts and amplitude variations are more likely to cause data error or transmission failures. These variations of optical performance will inevitably lead to fluctuations of the amplification characteristics of optical amplifiers.
The presence or absence of individual channels across the whole gain band has an important influence on the characterization of optical performance of optical amplifiers. In some cases, for example, a channel may be absent such that extra amplification of the other existing channels will result. It is obvious that as more channels are absent, channel amplification becomes a more serious problem.
It is therefore important to monitor the performance of an optical amplifier in an optical network, and in particular the individual channels. To do so, external channel performance monitors have been used in conjunction with optical amplifiers. A compact channel performance monitor is described in U.S. patent application Ser. No. 09/715,765 filed Nov. 17, 2000 titled COMPACT OPTICAL PERFORMANCE MONITOR, the contents of which are incorporated herein by reference. The channel performance monitor can be tailored and integrated into an optical amplifier.
The present invention provides a method and system of integrating optical amplifiers with a spectral monitor. The spectral gain monitor is a compact module having a low-cost volume phase grating (VPG) optical element, a compact photo-detector array and a micro-processor controller. It is initially designed for EDFAs, but not limited to.
A primary object of the present invention is to provide a compact design of a low-cost optical amplifier system with spectral gain monitoring capabilities based on erbium-doped fiber amplifiers and VPG technology. The present invention provides a method for designing optical amplifiers with spectral gain monitoring capabilities for Raman amplifiers and other semiconductor optical amplifiers. Accordingly, a method is provided for designing a multichannel device with spectral gain monitoring capabilities.
In the preferred embodiments of the present invention, an optical amplifier with spectral gain monitoring capabilities is provided wherein individual channel powers (including the presence or absence of some channels) are monitored. Feedback control to stabilize variation of optical performance is also provided.
In accordance with the present invention, there is provided a system for amplifying an input wavelength division multiplexed (WDM) optical signal with a first optical coupler operative to receive the input WDM optical signal and extract a portion of the signal therefrom. The system further includes a first spectral monitoring unit having a volume phase grating optically connected to the first coupler. The first spectral monitoring unit separates the input WDM optical signal into input spectral components (i.e., prescribed channels) and detects the power levels thereof. An optical amplifier is optically connected to the first coupler and amplifies the input WDM optical signal to generate an amplified output WDM optical signal. The optical amplifier may be a laser pump source optically connected to an erbium-doped fiber. A second optical coupler is optically connected to the optical amplifier and extracts a portion of the output WDM optical signal. The system has a second spectral monitoring unit with a volume phase grating optically connected to the second optical coupler. The second spectral monitoring unit separates the output WDM optical signal into output spectral components (i.e., prescribed channels) and detects the power levels thereof. A controller is electrically connected to the first spectral monitoring unit, the second spectral monitoring unit and the optical amplifier. The controller dynamically operates the amplifier in response to the power levels of the input and output spectral components. In this regard, it is possible for the amplifier to dynamically adjust the amplification of the input optical signal in response to the power in the channels.
The first and second spectral monitoring units separate and detect the power level in the spectral components of the extracted input and output signals. Accordingly, the spectral monitoring units each have an input fiber for receiving the optical signal and a collimating lens optically connected to the input fiber. The collimating lens emits the optical signal onto the volume phase grating which separates the optical signal into spectral components. Each of the first and second spectral monitoring units further include a focusing lens for focusing the spectral components onto a photo-detector array which detects the power level of each of the spectral components. The photo-detector array has a plurality of photo-detectors wherein each of the photo-detectors correspond to one of the spectral components. In this regard, each of the photo-detectors detects the power level of a respective one of the spectral components.
It will be recognized by those of ordinary skill in the art that the amplifier system may operate with only a single spectral monitoring unit. In this regard, the spectral monitoring unit will determine the power levels of each of the spectral components by processing the extracted input and output optical signals either in a serial manner or parallel manner. For instance, if the signals are processed in a serial manner, an optical switch will be used to switch between the extracted input and output signals. If the signals are processed in a parallel manner, the volume phase grating, as well as the photo-detector array, will be configured to receive both the extracted input and output optical signals simultaneously.
In accordance with the present invention, there is provided a method of amplifying an input optical signal with an optical amplifier system having a first and second optical coupler, a spectral monitoring unit, an optical amplifier, and a controller. The method starts by extracting a portion of the input WDM optical signal with the first optical coupler. Next, the input WDM optical signal is amplified with the optical amplifier in order to generate an output WDM optical signal. A portion of the amplified output WDM optical signal is extracted with the second optical coupler. The spectral monitoring unit separates the spectral components of the extracted input and output WDM signals and detects the power levels of the spectral components. The controller dynamically operates the optical amplifier in response to power levels of the spectral components. In this regard, the controller can control the amplification of the input WDM optical signal in order to provide uniform amplification.